A comet's chilly origin.

When Comet Hale-Bopp passed near the sun 3 years ago, it dazzled skywatchers with brilliant tails of dust and gas. The comet also vented traces of a precious cargo: the inert gas argon.

Scientists had never before detected argon or any other noble gas in a comet, notes S. Alan Stern of the Southwest Research Institute in Boulder, Colo. The presence of argon, he and his colleagues report, suggests Hale-Bopp was born in the outer solar system, between the orbits of Uranus and Neptune. Scientists had previously suspected that the comet arose from material in Jupiter's vicinity, which is closer to the sun.

Hale-Bopp spends most of its time in the deep freeze of the Oort cloud, a vast reservoir of comets hundreds of times farther from the sun than Pluto lies. Calculations indicate that the Oort cloud originated in a region that extends from the orbit of Neptune to that of Jupiter.

On the evening of March 29, 1997, a detector-laden rocket from White Sands, N.M., roared into space and took spectra of Hale-Bopp as it neared its closest approach to the sun. The flight lasted just 5 minutes, but because Hale-Bopp loomed big and bright, that was enough time for Stern and his colleagues to detect the ultraviolet fingerprint of argon.

The team reported the findings last month at a meeting of the American Astronomical Society in Rochester, N.Y.

Argon is a sensitive indicator of the temperature at which Hale-Bopp formed because it doesn't interact with other elements and boils away at a temperature of just 40 kelvins (K). Previously reported observations with another spacecraft, the Extreme Ultraviolet Explorer (EUVE), had found no sign that the comet contained another inert gas, neon, which boils at 25 K.

The detection of argon and the absence of neon indicates that Hale-Bopp formed in a region of the early solar system that has a temperature between 25 and 40 K. Jupiter's neighborhood is too warm, but the region between the orbits of Uranus and Neptune is in that temperature range, notes Michael J. Mumma, an astronomer at NASA's Goddard Space Flight Center in Greenbelt, Md., and a member of the EUVE team. One complication, he notes, is that the interpretation of the results depends on how the frozen argon is trapped inside Hale-Bopp.